The present invention relates to a method of dynamically filling containers with gas mixtures, particularly O2/N2O mixtures containing an N2O proportion not less than 30% by volume, at a pressure of at least 170 bar.
At the present time, there are several methods of filling pressurized containers, such as gas bottles, with gas mixtures.
Thus, the method referred to as gravimetric filling is generally used for filling with gas mixtures based on liquefied gases, such as N2O or CO2, or mixtures of air gases, such as O2, N2, Ar or He. However, this filling method has the drawbacks of resulting in a high level of manufacturing scrap, after analytical inspection, a low-productivity manufacturing process, since the containers must be filled one by one, a container rolling cycle that penalizes production times, and a high analytical inspection cost.
Moreover, the pressure/temperature gravimetric sequential filling method is also known. However, with this method, the mixtures produced in the various bottles from one and the same production rail often exhibit deviations in the final composition. To avoid this, it is necessary to comply with pressure stabilization and balancing times that penalize the overall productivity.
In the case of other conventional methods of filling containers with mixtures, the amounts of gas introduced are therefore controlled by measuring the pressure and the temperature of the gases. However, the determination of the gas contents is based on two measurement instruments, their measurement inaccuracies being additive. In addition, the location of the measurement points on the filling plant does not allow direct access to the physical quantities desired, i.e. the temperature and the pressure are generally measured on the filling rail by a temperature probe or a pressure sensor. However, the values thus measured are only approximations, not effective measurements of the temperature or the pressure within containers.
The method of mixing the gases dynamically partly overcomes these problems and drawbacks. This method, described for example in document EP-A-1 174 178, consists in filling the bottles with the gas mixture in its expected final composition from the start right to the end of the filling sequence. The mixture is produced upstream of the filling rail in a very small mixing chamber into which the various gaseous constituents making up the composition of the final mixture are introduced.
The amounts of each gas introduced are controlled by a mass flowmeter installed on the line for each constituent gas of the composition of the mixture to be produced. Moreover, a combination of several regulating valves is used to control the flow rate of the gases thanks to the action of an automatic regulating system. Mass metering by a mass flowmeter makes it possible to factor out any uncertainties in the measurements and any production vagaries associated with the inaccuracies as regards the amounts mentioned above.
However, filling with a dynamic mixer is accompanied, in certain cases, by expansion of the gas downstream of the mixing chamber and a lowering of the temperature of the gases below the demixing temperature, which is explained by the fact that the line downstream of the chamber is at the same pressure as the containers relative to atmospheric pressure. The gas flow is then a two-phase flow in the bottle-filling rails.
Given that the liquid and gaseous phases flow at different flow rates, the operation of filling the bottles is no longer uniform and deviations in the final contents may be observed in bottles filled from the same rail during one and the same manufacturing run. These disparities may be explained by preferential flows in the pipes of the container-filling rails.
To solve this demixing problem, document EP-A-1 174 178 has proposed to maintain the mixture above the demixing temperature by using, in order to do this, a perfectly regulated heater for heating the gases leaving the dynamic mixing chamber during the filling cycle.
Since the mixture is thus always maintained in the gaseous state, the homogeneity of the mixture is preserved and the deviations in contents are low enough to make it possible for the set of bottles to be checked by analyzing only a single bottle taken off the filling rail.
However, in practice, there is sometimes a limitation in filling containers with certain gas mixtures, in particular of the O2/N2O type in which the N2O content is not less than 30% by volume for pressures above 170 bar.
This is because, for this type of mixture, the final pressure is limited by the pressurization of the N2O to around 170 bar. The N2O must therefore be heated in order to rise to higher pressures, which then take it into the supercritical state.
The heating temperature is also limited by the decomposition temperature of N2O, the more so as certain metals of the filling device and the bottle, such as silver, platinum, cobalt, copper and nickel oxides, are catalysts for the reaction.
The dynamic filling of certain gas mixtures is therefore in general limited to a pressure of around 170 bar.
The problem to be solved is therefore how to improve the method of filling using a dynamic mixer, especially the method described by document EP-A-1 174 178, so as to be able to fill containers dynamically with gas mixtures at pressures above 170 bar, in particular medical gas mixtures of the N2O/O2 type, the N2O content of which is not less than 30% by volume.